Ion implantation techniques, e.g. for modifying the electrical conductivity properties of semiconductor materials, are known in the manufacture of integrated circuit structures in semiconductor wafers. Such ion implanters generally comprise an ion beam generator having a source of ions of the element to be implanted in the semiconductor wafer, and an extraction assembly for extracting ions from the source and forming a beam of the extracted ions. The ion beam so produced is then passed through a mass analyser and selector for selecting a particular species of ions in the ion beam for onward transmission for implantation in the wafer or target substrate.
In order to ensure good homogeneity of the dose of implanted ions over the surface of the target substrate being implanted, the substrate and the ion beam are scanned relative to each other so that in one process treatment the dose at any position on the target substrate is made up of components from a large number of individual scans. A common scanning technique used in the prior art involves mechanically moving the target substrate relative to a stationary beam. The target substrate is mechanically scanned in two dimensions. Apparatus of this kind is described for example in U.S. Pat. No. 5,389,793. In such apparatus, a number of target substrates are mounted on a rotatable wheel, whereby rotation of the wheel in the process chamber of the implant tool provides the scan of each of the target substrates mounted on the wheel in one direction, and reciprocating movement of the axis of rotation provides the scanning movement in a second direction.
The physical speed of relative scanning of target substrate and ion beam by mechanical means limits the minimum process time possible with such mechanical scanning implanters, if a certain minimum number of repeat scans is required for each substrate in order to ensure adequate homogeneity of dose over the substrate surface. In prior art machines, a batch of target substrates are processed simultaneously in order to provide adequate throughput for the implant machines.
Because the rotatable wheel of the process chamber is rotated during scanning at a high rate, the wafers are mounted on the wheel on mounting surfaces which are angled relative to the plane of rotation slightly inwards towards the axis of rotation. In this way, the wafers are held firmly against the wafer support surfaces by centrifugal force. The wafers are therefore distributed on the process wheel on a shallow conical surface and are scanned in this conical surface through the beam during processing. As a result, the angle between the beam and the wafer changes slightly as the wafer is scanned through the beam, resulting in a small non-uniformity of the angle of implantation of ions into the wafer.
Also, with a batch processing system as known in the prior art, if there are insufficient wafers available for processing to fill all the available mounting points on the process wheel, unfilled mounting points must be filled with dummy substrates to maintain the balance of the wheel. These dummy substrates are expensive and can cause process problems such as contamination, especially if the same dummy wafer is used repeatedly.
Also, any process failure occurring during a batch run in known batch processors results in all wafers in the batch being scrapped.
The above problems of batch processing ion implanters would be readily resolved by a machine arranged to process wafers singly. Single wafer processing would also fit in better with the single wafer processing tools customarily used in semiconductor fabrication plants for other processing procedures. However, mechanical scanning alone would not give the processing speeds necessary for a single wafer machine to provide adequate output.
Accordingly, single wafer implanters have been proposed in which the fast scanning of the ion beam relative to the wafer in one co-ordinate direction is performed by electrostatic or electromagnetic scanning of the beam, while the slower rate scanning in the orthogonal co-ordinate direction is performed by mechanically moving the wafer.
Although ion beams may be theoretically scanned either electrostatically or electromagnetically, there are well established problems with electrostatic scanning. It is well known to those experienced in this field that difficulties arise in controlling ion beams, especially at relatively lower energies, when the ion beam travels through regions of electric field. These problems arise because electrons cannot exist for substantial times in regions of electric field, so that beams of positive ions can experience severe space charge effects in the absence of neutralising electrons in such regions. As a result, the ion beam tends to "blow up" and become uncontrollable.
Existing proposals for ion beam scanning contemplate using varying magnetic fields to effect the scanning of the beam. In fact a first region of varying magnetic field provides angular deflection of the ion beam, which then passes into a second region of magnetic field to collimate the deflected beam, so that the beam at the target substrate strikes the substrate at a constant angle as it scans across the substrate. Existing proposals contemplate performing the magnetic scanning and magnetic collimation of the scanned beam after mass selection. A single wafer machine using magnetic beam scanning is disclosed in "Fast Magnetic Scanning and Ion Optical Features of the new IBIS Oxygen Implanter", by Glavish et.al., Ion Implant Technology--92, poublished by Elsevier in 1993.
The above described magnetic beam scanning proposal still occupies substantial space and the scanning magnet may be expensive and require expensive HF power amplifiers. Further, it is believed that electromagnetic scanning of the ion beam may also compromise the space charge neutralisation of the ion beam, especially in the scanning region. This may become especially a problem for relatively low energy ion beams.
There is thus a continuing need for scanning arrangements in ion implanters which can minimise the size of the resulting implanter and also reduce problems of maintaining space charge neutralisation of the ion beam.